This is fairly simple really. All fish have traits that correspond to the habitat they naturally live in. Whether it’s the temperature of the water or the lighting, fish are very particular about the water they’re swimming in and are prone to dieing when they get put into a tank that doesn’t fit they’re traits.

With an aquarium, we try to mimick these habitats so that the fish can comfortably life in our aquarium. Ideally, this is so the fish don’t die repeatedly on the owner. It would be a pain to keep purchasing new fish and very thoughtless to treat animals this way. Specifically, PH is the measure of the acidity of the water. Fish are generally used to a certain range or level of PH that your aquarium must match in order for that fish to survive.  That is why you have to check and test the water PH level of your fish tanks.